Explanation of “why stainless steel rusts”
First. Definition
“Stainless steel” is just a general term. The standard name is “acid and alkali resistant steel”. Stainless steel is a steel product made by adding metal elements such as chromium and nickel on the basis of ordinary steel. The content of metal elements such as nickel, molybdenum, and titanium The higher the quality, the better the quality of stainless steel.
Second, the characteristics of stainless steel
Stainless steel has the ability to resist atmospheric oxidation-that is, non-rust, and it also has the ability to resist corrosion in media containing acid, alkali, and salt-that is corrosion resistance. But the size of its anti-corrosion ability varies with the chemical composition of the steel itself, the state of mutual addition, the conditions of use, and the type of environmental media.
For example, 304 steel pipe has an absolutely excellent anti-corrosion ability in a dry and clean atmosphere, but when it is moved to a coastal area, it will quickly rust in sea fog containing a lot of salt; while 316 steel pipe has performance good.
Therefore, it is not any kind of stainless steel, which can resist corrosion and rust in any environment.
Three, the cause of stainless steel rust
Stainless steel relies on a very thin, strong, dense, and stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to penetrate and continue to oxidize, thereby obtaining the ability to resist corrosion. Once for some reason, this film is continuously destroyed, oxygen atoms in the air or liquid will continue to infiltrate or iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will continue to be rusted. There are many forms of damage to this surface film, and the following are more common in daily life:
Stainless Steel Rust
1. The surface of stainless steel has accumulated dust containing other metal elements or attachments of foreign metal particles. In humid air, the condensation between the attachments and stainless steel connects the two into a micro battery, which initiates an electrochemical reaction. , The protective film is damaged, which is called electrochemical corrosion.
2. The surface of stainless steel adheres to the organic juice (such as melon, vegetable, noodle soup, sputum, etc.), which forms organic acid in the presence of water and oxygen, and the organic acid will corrode the metal surface for a long time.
3. The surface of stainless steel adheres to containing acid, alkali, and salt substances (such as alkaline water and lime water splashing on the decoration wall), causing local corrosion.
4. In polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide, and nitrogen oxide), it will form sulfuric acid, nitric acid, and vinegar when it encounters condensed water.
Fourth, the form of rust
There are mainly the following forms: stress corrosion cracking, pitting corrosion, intergranular corrosion, crevice corrosion, and general corrosion
1. Stress corrosion cracking (SCC): refers to a kind of mutual failure of stress-bearing alloys due to the expansion of streaks in a corrosive environment
2. Pitting corrosion: Pitting corrosion refers to a high degree of local corrosion that does not corrode or is slightly corroded on the surface of metal materials. The size of common corrosion spots is less than 1.00mm, and the depth is often greater than the surface aperture. The lighter ones have shallower corrosion. Pit, serious, or even perforation.
3. Intergranular corrosion: selective corrosion damage, which is different from general selective corrosion in that the locality of corrosion is on the microscopic scale.
4. Crevice corrosion: It refers to spot-like or ulcer-shaped macroscopic corrosion pits in the gaps of metal components. It is a form of local corrosion. It may occur in the gaps where the solution stagnates or in the shielded surface. Such gaps can be formed at the junction of metal and metal or metal and non-metal.
5. Comprehensive corrosion: it is used to describe the corrosion that occurs on the entire alloy surface in a relatively uniform manner
Five, the principle of no rust
Stainless steel can not rust because it contains chromium. Usually, the content of chromium must reach 13-25% (weight ratio) in order to change the oxidation of the steel surface.
Principle: After adding chromium and other elements to steel, the properties of steel can be changed. For example, the molecular structure of steel is more uniform, and it is easier to form a dense oxide protective film on the surface of the steel, thereby greatly improving the corrosion resistance of stainless steel.
Therefore, stainless steel can resist the corrosion of fire, water, acid, alkali, and various solutions, and does not rust. Scientists found that the more uniform the internal structure of steel, the more closely the various components are connected, the more difficult it is for corrosion products to invade, and that there is a layer of oxide protective film attached to the surface, just like putting armor on steel. It will not rust easily.
Six. Problems that should be paid attention to in use
1. Protect the chromium-rich oxide film (protective film). Avoid damage caused by scratches, deposits, and bleach, otherwise, stainless steel will rust.
2. Pay attention to small pits and cracks on the surface, because this is the beginning of rust.
3. Use appropriate tools. Do not use steel wipes, wire brushes, or scrapers.
4. Use chlorinated alkali detergent according to the recommended strength
5. Wipe off the suspended detergent and residual moisture immediately, otherwise it will eventually cause rust if it stays on the stainless steel for a long time